rename privacy "private" → "unlisted"; enable GPS for unlisted

- "unlisted" = not shown in the public feed, but GPS track, timeseries
  and detail JSON are all accessible by direct URL (security by obscurity)
- "private" accepted as legacy alias everywhere (backward compat with
  existing data on disk)
- New writes from Strava sync / ZIP upload / sidecar use "unlisted"
- Only "no_gps" now suppresses the GPS track
- isUnlisted() helper in format.ts used by all Svelte/Astro components
- SCHEMA.md and CLAUDE.md document the privacy model and the distinction
  between "unlisted" and "no_gps"

bincio/render/merge.py

@@ -49,7 +49,7 @@ def apply_sidecar(detail: dict, fm: dict, body: str) -> dict:
     if "highlight" in fm:
         d["custom"]["highlight"] = bool(fm["highlight"])
     if "private" in fm:
-        d["privacy"] = "private" if fm["private"] else detail.get("privacy", "public")
+        d["privacy"] = "unlisted" if fm["private"] else detail.get("privacy", "public")
     if "hide_stats" in fm:
         d["custom"]["hide_stats"] = [str(s) for s in (fm["hide_stats"] or [])]
 
@@ -69,7 +69,7 @@ def _apply_sidecar_summary(summary: dict, fm: dict) -> dict:
     if "highlight" in fm:
         s["custom"]["highlight"] = bool(fm["highlight"])
     if "private" in fm:
-        s["privacy"] = "private" if fm["private"] else summary.get("privacy", "public")
+        s["privacy"] = "unlisted" if fm["private"] else summary.get("privacy", "public")
 
     return s
 
@@ -260,10 +260,10 @@ def merge_all(data_dir: Path) -> int:
                 s = _apply_sidecar_summary(s, fm)
             activities.append(s)
 
-        # Drop private activities from the published feed
-        # Sort: newest first, then bring highlighted activities to the top
-        # Private activities are kept in the index so the owner can see them;
-        # the feed UI filters them out for non-owners client-side.
+        # "unlisted" (and legacy "private") activities are kept in the index so
+        # the owner can reach them by direct URL; the feed UI filters them out
+        # for non-owners client-side.
+        # Sort: newest first, then bring highlighted activities to the top.
         activities.sort(key=lambda a: a.get("started_at", ""), reverse=True)
         activities.sort(key=lambda a: 0 if a.get("custom", {}).get("highlight") else 1)